4 ms·

> There is a lot of assumptions that are broken if you try to allocate something smaller than /64. It would be like trying to re-assign the multicast address sp

> There is a lot of assumptions that are broken if you try to allocate something smaller than /64. It would be like trying to re-assign the multicast address space for unicast. Can you go into some details as to what breaks? I ran a dual-stack network with /112s for host subnets, and /124s for ISLs (prefix lengths chosen to make life easier for the network operators) and I wasn't aware of any problems. Certainly, you can't use (formerly known as) class D address space in IPv4 as the major IPv4 stacks will balk if you do, and you're never gonna get into class E space either for similar reasons, but ... to use a longer prefix length than /64 in IPv6? I'm unaware of any implementations that fall over, but it's been a few years since I paid any real attention.

zamadatix 5y ago
For what breaks: a lot. This has a decent start of the list https://datatracker.ietf.org/doc/html/rfc5375 https://datatracker.ietf.org/doc/html/rfc5375 > Using a subnet prefix length other than a /64 will break many features of IPv6, including Neighbor Discovery (ND), Secure Neighbor Discovery (SEND) [RFC3971], privacy extensions [RFC4941], parts of Mobile IPv6 [RFC4866], Protocol Independent Multicast - Sparse Mode (PIM-SM) with Embedded-RP [RFC3956], and Site Multihoming by IPv6 Intermediation (SHIM6) [SHIM6], among others. A number of other features currently in development, or being proposed, also rely on /64 subnet prefixes. SLAAC is part of that as well. > I ran a dual-stack network with /112s for host subnets, and /124s for ISLs (prefix lengths chosen to make life easier for the network operators) and I wasn't aware of any problems. Easiest for network operators is to act like IPv6 doesn't have different subnet mask at all, assume every subnet is always /64 and never have to remember funny patterns. Even a point to point is most often best done as a /64 instead of a /127. Most switching hardware prefers this as well actually, by ignoring the last 64 bits of the address in routing the hardware scales significantly better in terms of TCAM usage.

zamadatix 5y ago

For what breaks: a lot. This has a decent start of the list https://datatracker.ietf.org/doc/html/rfc5375 https://datatracker.ietf.org/doc/html/rfc5375 > Using a subnet prefix length other than a /64 will break many features of IPv6, including Neighbor Discovery (ND), Secure Neighbor Discovery (SEND) [RFC3971], privacy extensions [RFC4941], parts of Mobile IPv6 [RFC4866], Protocol Independent Multicast - Sparse Mode (PIM-SM) with Embedded-RP [RFC3956], and Site Multihoming by IPv6 Intermediation (SHIM6) [SHIM6], among others. A number of other features currently in development, or being proposed, also rely on /64 subnet prefixes. SLAAC is part of that as well. > I ran a dual-stack network with /112s for host subnets, and /124s for ISLs (prefix lengths chosen to make life easier for the network operators) and I wasn't aware of any problems. Easiest for network operators is to act like IPv6 doesn't have different subnet mask at all, assume every subnet is always /64 and never have to remember funny patterns. Even a point to point is most often best done as a /64 instead of a /127. Most switching hardware prefers this as well actually, by ignoring the last 64 bits of the address in routing the hardware scales significantly better in terms of TCAM usage.